\documentclass[11pt]{article}

\begin{document}

\begin{theorem}
\declare{syntax={infix, 7, "/", l}, meaning=arith1.divide}
\end{theorem}
\begin{proof}
\begin{lemma}
\declare{syntax={infix, 7, "/", l}, meaning=arith1.divide}
\end{lemma}
\end{proof}

\end{document}